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Analysis of tungsten carbide coatings by UV laser ablation inductively coupled plasma atomic emission spectrometry
Spectrochimica Acta Part B 55 Ž2000. 575᎐586
Analysis of tungsten carbide coatings by UV laser ablation inductively coupled plasma atomic emission spectrometry V. Kanicky a,U , V. Otrubaa , J.-M. Mermet b a
Laboratory of Plasma Sources for Chemical Analysis, Faculty of Sciences, Masaryk Uni¨ ersity Brno, Kotlarska 2, CZ 61137 Brno, Czech Republic b Laboratoire des Sciences Analytiques (CNRS UMR 5619), Uni¨ ersite´ Claude Bernard-Lyon I, Batiment 308, F 69622 Villeurbanne, France Received 3 December 1999; accepted 15 February 2000
Abstract Tungsten carbide coatings Žthickness 0.1᎐0.2 mm. containing 8.0, 12.2, 17.2 and 22.9% Co were studied with laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES.. Composition of these plasma sprayed deposits on steel disks was determined using X-ray fluorescence spectrometry and electron microprobe energyrwavelength dispersive X-ray spectrometry. The coatings were ablated by means of a Q-switched Nd:YAG laser at 266 nm Ž10 Hz, 10 mJ per shot. coupled to an ICP echelle-based spectrometer equipped with a segmented charge-coupled device detector. Non-linear dependences of cobalt lines intensities on the Co percentage were observed both at a single spot ablation and at a sample translation. This behaviour could be attributed to a complex phase composition of the system W᎐C᎐Co. However, employing tungsten as internal standard the linear calibration was obtained for studied analytical lines Co II 228.616 nm, Co II 230.786 nm, Co II 236.379 nm and Co II 238.892 nm. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: UV laser ablation; Tungsten carbide coatings; Inductively coupled plasma; Atomic emission spectrometry
U
Corresponding author. Tel.: q42-5-41129283; fax: q42-5-41211214. E-mail address: [email protected] ŽV. Kanicky.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 1 6 7 - 1
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1. Introduction Over two past decades, laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES. has become an efficient tool for direct analysis of compact solids, as a laser beam allows vaporisation of chemically resistant materials, both conducting and non-conducting. Since the mid-1960s, composite materials, or structural ceramics, have been gaining increased importance in various production technologies. Advanced ceramics are now finding widespread use in many load-bearing applications, such as cutting tools, car-engine components, heat exchangers, turbines, wear parts, and aerospace, where they are applied as protective coatings w1x. Determination of chemical composition of these coatings might be a particular task for the laser ablation-based spectrometry. Recently, LA-ICP-AES was applied to the determination of Zr and Ti in 3-m-thick ZrTiN coatings, which were prepared by physical vapour deposition process on a high-speed steel substrate w2,3x. Nitrided steel surfaces with 5-m layers of sputtered Ti or V were investigated using frequency-tripled and frequency-quadrupled Qswitched Nd:YAG laser coupled to an ICP-AES w4x. Qualitative depth profiles of zirconia graded metal-ceramic coatings of thickness of 700 m, prepared by plasma-spraying on steel substrate, were obtained by means of a frequency-quadrupled, pulsed Nd:YAG laser with an ICP emission spectrometer w5x. Some high-tech ceramic materials, such as ceramic materials for electronic w6x, and ceramic layers of solid oxide fuel cells w7x were analyzed using LA with ICP mass spectrometry ŽICP-MS.. Thermal spraying of ceramic coatings represents very useful way of surface treatment of metal parts and tools that are exposed to extreme mechanical loading, thermal shocks andror chemical attacks w8x. Information on homogeneity and chemical composition of these ceramic layers is important for optimization of production technology. Being highly chemically resistant, these coatings are usually examined by means of techniques of surface analysis, such as glow discharge optical emission spectrometry, secondary ion mass
spectrometry, X-ray fluorescence spectrometry, and electron microprobe energy dispersive X-ray spectrometry. However, these techniques allow to obtain chemical information restricted only to near-surface layers, while the thickness of the coatings such as partially stabilized ZrO 2 thermal barrier coatings w9,10x or hydroxyapatite composite coatings for bioceramics w11,12x is approximately several hundreds of micrometers. In such cases, average in-depth composition could be possibly determined by means of LA-ICP spectrometry. The aim of this work was to study the possibility of using LA-ICP-AES ablation for the determination of cobalt content in tungsten carbide ŽWC-Co. coatings, which were sprayed on steel substrates by means of a microprocessor-controlled plasmatron fed with WC powders containing different percentages of Co. The critical parameter was expected to be the homogeneity of coatings and different physical characteristics of Co and tungsten carbide. Therefore, attention was paid to evolution of temporal LA-ICP-AES signals, correlation of Co and W line intensities and internal standardization using tungsten lines. Reproducibility of ablation, linearity of calibration, and precision and accuracy of determination were investigated using tungsten carbide samples with four different percentages of cobalt.
2. Experimental 2.1. Instruments and operating conditions A Q-switched Nd:YAG laser Žmodel Surelite Ir20, Continuum, USA. was used for ablation at its 4th harmonics Ž266 nm. with the laser pulse width of 5 ns and the laser pulse repetition rate of 10 Hz. The pulse energy of 10 mJ was adjusted by setting the Xe-flashlamp discharge voltage. The beam expander ŽModel BXG0-10.0-3X-266, CVI Laser Corporation, USA. was located between the laser and the focusing plano-convex quartz lens with a 270-mm focal length in the visible range. The beam expander output lens was 150 mm in front of this lens. The optimum focusing was achieved when observing the highest acoustic
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emission during the ablation process and the resulting laser spot diameter was approximately 0.1 mm. The average power was measured with an Astral TM Laser PowerrEnergy Meter, Model AD 30 ŽScientech, USA.. The ablation cell consisted of a sample holder and a glass chamber of the volume of 140 cm3, provided with a silica window. The carrier gas Ž0.65 dm3 miny1 Ar. was introduced through a gas inlet into the chamber in the parallel direction to the sample surface. The gas outlet was situated opposite to the inlet. The ablated material was transported by the carrier argon flow along a polyamide tubing Žlength 1.5 m, i.d. 4 mm. through the standard double-pass Scott spray chamber to the ICP. The cell was mounted on a computer controlled, motor-driven XY-translation stage ŽModel PCC 2r4, Schneeberger, France. to be moved with respect to the laser beam which was perpendicular to the target surface. Both fixed-spot ablation and ablation with translation along the 1 mm= 1 mm-square pattern were carried out. The translation speed was always 1 mm sy1 . A Perkin-Elmer Optima 3000 ICP Dual View system was used in the axial observation mode for the analytical signal measurements. The 40-MHz free-running generator was operated at the forward power of 1.1 kW, and the outer, intermediate and carrier gas flow rates were 15.0, 0.5 and 0.65 dm3 miny1 of argon, respectively. The carrier gas flow rate was optimized to obtain maximum signal-to-background ratio. The flat maximum between 0.6 and 0.7 dm3 miny1 was found. The polychromator with an echelle grating was provided with segmented-array charge-coupled device detectors ŽSCD.. The spectral bandwidth of the polychromator is 0.007 nm at 220᎐240 nm w13x. This system allowed simultaneous measurement of several spectral lines including their off-peak background correction positions. The sum of three adjacent pixels of the SCD was taken to measure the gross emission intensity Ža peak area mode.. This total intensity was corrected for the off-peak background emission. The sampling time of 1 s and integration times in the 5᎐200-ms range depending on the line intensity were employed for all measurements. On average, a signal was available every 2.4 s, including the
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sampling time and the detector reading and transfer time. The following analytical lines were measured Žwavelength in nm.: Co II 228.616; Co II 230.786; Co II 236.379; Co II 238.892; W II 207.911; W II 224.875; W II 248.924; Fe II 238.204 and C I 193.018. Tungsten carbide represents a difficult sample matrix from the viewpoint of both the possible spectral and non-spectral interferences. A great attention was paid to the selection of background correction wavelength positions. The following possible mutual spectral influences were expected: Co II 228.616 nm could be influenced by W 228.629 nm and W 228.590 nm; Co II 230.786 nm by W II 230.793 nm and W 230.766 nm; Co II 236.379 nm by W 236.389 nm; Co II 238.892 by W 238.880 nm and W 238.907; and vice versa, W II 207.911 nm could be influenced by Co I 207.931 nm; W II 224.875 nm by Co II 224.866 nm; W II 248.923 nm by Co II 248.925 nm. Overcorrection was observed for Co II 236.379 nm due to the interference of W II 236.389 nm on the background correction position at the longer wavelength, and therefore, only correction position at the shorter wavelength was used. Spectral interference of Fe appeared after penetrating of the laser beam into the steel substrate. The short-wavelength background correction position at 238.870 nm of the Co II 238.892 nm was overlapped by Fe II 238.868 nm. Therefore, only long-wavelength background position Ž238.921 nm. was used for correction. In other cases no spectral interferences were observed. 2.2. Samples Samples of tungsten carbide coatings containing cobalt were prepared by means of a commercial microprocessor-controlled atmospheric plasma spraying system ŽSulzer Plasma-Technik AG, Switzerland. in the facility of Plasmametal, Co. Ltd., Brno, Czech Republic. Following deposition parameters were employed: electrical power 28 kW, discharge current 600 A, plasma gas flow rate 42 dm3 miny1 of Ar, spraying distance 130 mm. Hydrogen Ž3 dm3 miny1 . was being added to the plasma argon flow during the spraying. Four powdered samples of tungsten carbide with
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declared percentages of Co Ž6%, 12%, 17% and 20%. were plasma-sprayed on steel disks with diameter of 58 mm and thickness of 1.5 mm. To find the exact value of the coating thickness, the sample was cut using the diamond circular saw ŽDepartment of Mineralogy, Petrology and Geochemistry, Masaryk University Brno. and the width of the coating was measured with Abbe comparator Žmade by Carl Zeiss Jena, Germany.. The thickness of coatings was in the range 0.10᎐0.17 mm. At each percentage of cobalt, three coated disks were prepared under identical conditions with the aim of studying the sample preparation reproducibility. The influence of the hydrogen flow rate was studied on samples containing 17% of Co. For this purpose, three disks were prepared for each of additional flow rates: 8 dm3 miny1 of H 2 and 13 dm3 miny1 of H 2 . Basic characteristics of WC᎐Co powders and thickness of coatings are given in Table 1a. A sample prepared independently in another laboratory ŽFaculty of Mechanical Engineering, Technical University Brno, Czech Republic. was used as an unknown sample for analysis. This sample consisted of WCq 12% Co top coating, which was plasma-sprayed on intermediate binding coat Ž20% Nir80% Al.. The total thickness of the combined deposit was 0.24 mm. Chemical composition of plasma sprayed coatings was determined by wavelength-dispersive X-ray fluorescence spectrometry ŽXRF. and electron microprobe-energyrwavelength-dispersive X-ray spectrometry ŽEDXrWDX.. In order to match the sample size to dimensions of the XRF and EDXrWDX sample compartments, one disk for each Co percentage value was cut into four parts that were embedded with polymethyl-
metacrylate ŽPMM.. Four disks of the diameter of 40 mm were obtained in this way. The samples were cleaned after cutting in the ultrasonic bath using ethanol. The XRF and EDXrWDX analyses were carried out at each of the four PMMembedded samples for all Co percentages. The XRF analysis was performed using the sequential spectrometer PW 1404 ŽPhilips, the Netherlands. equipped with a rhodium target X-ray tube and the LiF 220, Ge111 and TlAP analyzer crystals. Percentages were calculated by means of a UNIQUANT software ŽPhilips., a precalibrated analytical program for semiquantitative analysis of unknown samples. UNIQUANT determines the net intensities of fluorescence lines and corrects for matrix interelement effects using ␣-coefficients computed with the method of fundamental parameters. The samples were analyzed at the Research Institute of Inorganic Chemistry, ´ stı´ nad Labem, Czech Republic. U The EDXrWDX analysis was performed with the electron scanning microscope system CamScan 4DD ŽCambridge Instruments, UK. coupled to the energy-dispersive X-ray analyzer LINK AN 10000 and wavelength-dispersive X-ray analyzer MICROSPEC 2A. Analyzers were calibrated with 100% W, 100% C and 100% Co. The results were normalised to the sum of 100%. These analyses were carried out at the Department of Mineralogy, Petrology and Geochemistry, Masaryk University Brno, Czech Republic. Results of XRF and EDXrWDX determinations are presented in Table 1b. Confidence intervals st n,1y␣ of the mean values of percentages were calculated based on four determinations. Iron was detected as the main impurity in all samples. It follows from presented data that no
Table 1a Description of the tungsten carbidercobalt powders and thickness of plasma sprayed coatings Material
Declared powder composition
Grain size Žm.
Coating thickness Žmm.
Metco Metco 71 VF NS PT 7446 PT T 5006
WCq 6% Co WCq 12% Co WCq 17% Co WCq 20% Co
y45 q 5 y45 q 5 y53 q 5 y45 q 11
0.12 0.12 0.10 0.17
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Table 1b Chemical composition of the plasma sprayed coatings determined with wavelength dispersive X-ray fluorescence spectrometry ŽXRF. and electron microprobe-energyrwavelength-dispersive X-ray spectrometry ŽEDXrWDX. a Material
Metco Metco 71 VF NS PT 7446 PT T 5006
XRF
EDXrWDX
Co Žwt.%.
W Žwt.%.
c
stn,1y␣
c
stn,1y␣
8.0 12.2 17.2 22.9
0.2 0.3 0.3 0.3
82.0 81.0 76.8 70.6
0.8 0.6 0.6 0.6
Fe Žwt.%. c
Co Žwt.%.
W Žwt.%.
c
stn,1y␣
c
stn,1y␣
3.5 1.9 0.9 1.0
7.8 11.5 17.1 22.6
0.3 0.4 0.6 1.2
79.3 78.8 77.5 72.0
0.7 1.5 1.6 1.0
Fe Žwt.%. c 4.2 1.5 0.6 0.9
a Confidence intervals st n,1y␣ are calculated at the level of significance ␣ s 0.05 for number of repetitions n s 4; wt.% means weight percentage.
statistically significant differences exist between results of both methods.
3. Results and discussion 3.1. Temporal beha¨ iour of signals Fig. 1 presents the temporal behaviour of signals of Co, W and C during ablation of the WC sample containing 8% Co ŽTable 1b.. Under translation, the intensities of W II 224.875 nm, Co II 228.616 nm and C I 193.018 nm lines reached the plateau after approximately 120 s and remained practically constant at least for 8 min Žcurves 1, 2 and 3 in Fig. 1.. Identical curves were obtained for all other cobalt and tungsten lines. Simultaneously, iron impurities within the coating were detected Žcurve 4, Fig. 1. and thus the results obtained by both X-ray methods were qualitatively proved. At the fixed-spot ablation, all the signals attained lower values in comparison to the translation experiment and the rapid decrease of intensities with time was observed after reaching their maxima Žcurves 5, 6 and 7 in Fig. 1.. This corresponds well to observations already described, e.g. at the ablation of partially stabilized zirconia coatings w5x and it could be possibly explained by hindered releasing of vaporized material from the single pit. Evolution of temporal signals during the fixed-spot ablation was less reproducible than under translation of sample, which might be due to local inhomogeneities. At the fixed-spot abla-
tion, the coatingrsubstrate interface was reached after approximately 2 min which implied from the steep increase of the Fe II 238.204 nm line intensity Žcurve 8, Fig. 1.. The non-zero signal of the iron line, which occurred before reaching the interface, confirmed the presence of Fe in the substrate. However, the increasing parts of all the temporal signals did not represent the true time courses of the laser᎐coating or laser᎐interface interactions, since the large dead volume of the ablation cell contributed to the rise-time of temporal signals. Besides this, the ablation of the coating continued even after penetrating of the beam into the substrate, because of conical shape of the crater.
Fig. 1. Laser ablation-ICP-AES temporal signals at the sample translation Žcurve 1, Co II 228.616 nm; 2, W II 224.875 nm; 3, C I 193.018 nm; 4, Fe II 238.868 nm. and at the fixed-spot ablation Žcurve 5, Co II 228.616 nm; 6, W II 224.875 nm; 7, C I 193.018 nm; 8, Fe II 238.868 nm..
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Although the signal of carbon exhibited a magnitude far above the background level, no analytical use was made of it because of a high ‘blank value’ and a monotonous content of carbon Žapprox. 5% C. in our samples. The curves 3 and 7 in Fig. 1 represent signals of the C 193.018 nm line, corrected both for the spectral background and the line intensity resulting from the presence of CO 2 in the ambient air. 3.2. Internal standardization and repeatability The possibility to use a major constituent of a sample matrix as an internal standard is of a great significance for improving precision, repeatability, reproducibility and accuracy of laser ablation analysis. The terms of repeatability and reproducibility used here are consistent with those introduced in w14x. In that paper the repeatability was estimated by dividing a temporal ablation signal, corresponding to one line pattern ablated in a sample, into 10 parts and calculating the relative standard deviation between the 10 mean values obtained w14x. In case of a tungsten carbidercobalt system, a complex behaviour at a laser᎐material interaction could be expected due existing numerous phases of different properties and composition, such as WC, W2 C, WC 1yx , Co 3 C, Co 2 C, Co 7W6 , Co 3W3 C, Co 3W9 C, Co 3W9 C 4 , Co 6W6 C w15᎐20x. Moreover, Co and W themselves differ substantially in melting Žm.t.. and boiling Žb.t.. temperatures: m.t.s 1490⬚C and 3400⬚C, b.t.s 3100⬚C and 5700⬚C, for Co and W, respectively w15x. For evaluating the possibility of internal standardization by tungsten, the ratios of the temporal signals presented in Fig. 1 were plotted in Fig. 2. At first, the signals obtained by ablation of a translated sample are discussed. It is obvious from the curve 1 in Fig. 2, that the intensity ratio of Co II 228.616 nm and W II 224.875 nm lines remained constant over the ablation period of 500 s after the rise-time Ž100 s.. Furthermore, the fluctuations of this ratio were considerably smaller in comparison to individual line intensities, see curves 1 and 2 in Fig. 1. The coefficient of correlation r between 83 data points for Co II 228.616 nm and W II 224.875 nm lines over the time
Fig. 2. Temporal behaviour of LA-ICP-AES intensity ratios at the sample translation Žcurves 1, CorW; 2, CrW. and at the fixed-spot ablation Žcurves 3, CorW; 4, CrW.. Employed lines: Co II 228.616 nm, W II 224.875 nm, C I 193.018 nm.
period from 200 s to 400 s was equal to 0.9792. Consequently, the relative standard deviation Ž%R.S.D.. of the signal ratios of Co and W within this period was 0.60%, while the %R.S.D. of the Co signals without internal standardization to W was 2.65%. This means that using internal standardization the signal stability was increased, or in other words, repeatability as defined in w14x was improved by a factor of 4.4. Surprisingly, a slight improvement by internal standardization was observed even for C I 193.018 nm line. The rising part of the curve 3 in Fig. 1 was straightened, as it is obvious from curve 2 in Fig. 2. However, the moderate increase of the carbon signal after 300 s of ablation was not entirely eliminated, compare with curve 3 in Fig. 1. Within the same period as for cobalt, the coefficient of correlation between C I 193.018 nm and W II 224.875 nm line intensities reached only the value 0.5952. Accordingly, the %R.S.D. of the C IrW II intensity ratio remained the same as the %R.S.D. of the carbon signal itself, i.e. 2.8%, and no improvement of repeatability was observed. It can be supposed that the correlation carbon᎐tungsten would be better if the blank signal of carbon were substantially lower than the signal originating in the sample. A more striking effect was observed at the fixed-spot ablation. In this case, the steep drop of both the Co II 228.616 nm and C I 193.018 nm
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line intensities Žcurves 5 and 7 in Fig. 1. was compensated by W II 224.875 nm line intensity, see curves 3 and 4 in Fig. 2. This straightening of curves continued even beyond the coatingrsteel interface. All the described features of LA-ICP-AES signals were found for other studied spectral lines of W and Co, for other WC᎐Co coatings and for each concentration level of cobalt. 3.3. Influence of crater depth on magnitude of LAICP-AES signals and estimation of ablation rate The diminution of the LA-ICP signals as a consequence of increasing depth of the ablation crater was demonstrated also by the behaviour of the signal of Fe originating from the coatingrsubstrate interface. The fixed-spot ablation was performed on samples with different coating thickness ŽTable 1a. and the signal of Fe was measured after reaching the interface. The decrease of the Fe II 238.204 nm line intensity with the increasing thickness of the WC᎐Co coating is plotted in Fig. 3, see curve 1. The error bars represent standard deviations based on four replicates. As the composition of the steel disks was identical, the lowering of the signal of Fe was due to a restraint caused by the narrow and deep crater to the release and transport of ablated material from the ablation spot. These obstructions to the aerodynamic flow have been already described when a depth-profiling of thick zirconia ceramic coatings was performed w5x. The time elapsed from the beginning of ablation to reaching the steel substrate was measured and plotted against the coating thickness Žcurve 2 in Fig. 3.. The linear regression yielded the equation, Time Žs. s 1173 = Thickness Žmm., with the coefficient of correlation r s 0.9805. The intercept was statistically insignificant. The ablation rate averaged over the coating depth in terms of the linear velocity of penetration was therefore constant within the experimentally studied thickness up to 0.17 mm, within the experimental errors and for four different coating compositions. This average ablation rate was equal to 0.87 m sy1 , or 0.087 m per shot.
Fig. 3. Influence of crater depth on magnitude of LA-ICP-AES signals and estimation of ablation rate. Dependence of the Fe II 238.868 nm line intensity originating from the ablation of the steel substrate on the thickness of the WC᎐Co coating at the fixed-spot ablation Žcurve 1.. Dependence of the time of penetration of the laser beam to the coatingrsubstrate interface on the coating thickness Žcurve 2. for the estimation of the ablation rate.
3.4. Reproducibility of ablation and homogeneity of samples Due to the technology of preparation, the WC᎐Co coatings were expected to be inhomogeneous to some extent, and therefore, inappropriate for microsampling by a laser beam. To find possible sources of uncertainties and to evaluate limitations for quantitative analysis, the ablation measurements were performed on all disks for a given percentage of cobalt and for each percentage level. The fixed-spot ablation was repeated five times on each disk. The laser interaction time was always 100 s to avoid the ablation of the coatingrsubstrate interface. Owing to the shapes of the fixed-spot temporal signals ŽFig. 1, curves 5, 6. the sum of replicates of ICP measurements that were obtained during the interaction time was taken as the analytical signal. Consequently, this analytical signal represented the average in-depth composition. Reproducibility of the analytical signal for a single disk was evaluated by calculating the %R.S.D. between the analytical signals recorded for five individual ablation pits in one disk, i.e. as already defined in w14x. Reproducibility of the analytical signal for the set of disks was evaluated by calculating the %R.S.D. between the analytical signals recorded for three
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ablation pits, each obtained on individual disk. This %R.S.D. for both the Co and W analytical signals was in the range from 7 to 15% regardless of the fact whether it was calculated within a single disk or between disks. Such errors seemed to be unacceptable for further evaluation and for that reason the CorW signal ratio was only used for following tests. The data for four spectral lines of Co and the W II 224.875 nm line as the internal standard were tested using the analysis of variance ŽANOVA.. From the ANOVA results it followed that the spread of CorW signal ratios within the disks did not differ significantly from the spread between the disks. As an example, the ANOVA results for the Co II 228.616 nm line are presented in Table 2. It is evident that calculated F-values are lower than the critical value Fcrit Ž 1 , 2 . of the Fisher᎐Snedecor distribution. The %R.S.D. values of the reproducibility of CorW signal ratios were within the range from 1.5 to 4% for all percentages of Co and also for all Co lines. Therefore, it could be concluded that the reproducibility within a single sample as well as between the samples of the same composition or even between different percentage levels is acceptable for quantitative analysis, with the relative standard deviation better than 4%. As we had no alternative WC᎐Co samples prepared with different technology providing high degree of homogeneity, we could not evaluate the reproducibility
of the ablation itself. Consequently, we can only conclude that the homogeneity of the plasma sprayed coatings corresponds with value of R.S.D. lower than 4%. 3.5. Influence of preparation conditions of coatings on analytical signal By the plasma spraying of the cermet coatings, hydrogen is usually added to the plasma gas flow of argon for increasing the heat transfer to particles of sprayed materials. The proportion of H 2 to Ar influences also the focusing of separate impingement particle spots on the substrate w9x. This could affect the properties of the coating. The influence of the H 2 flow rate on the LA-ICPAES signal ratio of CorW was investigated at three H 2 flow rates: 3, 8 and 13 dm3 miny1 . For each H 2 flow rate, three sprayed disks ŽWCr17% Co. were prepared. Fixed-spot ablation was performed on each disk and the data were evaluated using ANOVA. Average values and relative standard deviations calculated based on three disks for each level of hydrogen flow rate are given in Table 3. The decrease of LA-ICP-AES intensity ratio of CorW with increasing H 2 flow rate is statistically significant, because calculated F-value is equal to 17.35, and therefore substantially greater than the critical value Fcrit Ž2, 6. s 5.14 of the Fisher᎐Snedecor distribution. 3.6. Study of calibration
Table 2 Results of the analysis of variance for evaluation of the reproducibility of ablation a %Co
F
%R.S.D.
8.0 12.2 17.2 22.9
1.03 0.28 3.55 1.77
3.2 3.8 1.7 3.1
a
Evaluated signals: intensity ratios Co 228.616 nmrW 224.875 nm. Number of repetitions per disk, ps 5; number of disks per percentage of cobalt, n s 3; the critical value Fcrit Ž 1 , 2 . of the Fisher᎐Snedecor distribution for 1 s n y 1, 2 s n = py n, Fcrit Ž2, 12. s 3.885, the calculated value of F, the relative standard deviation %R.S.D. of a single observation calculated using n = p observations, i.e. the %R.S.D. of the reproducibility of ablation.
Calibration curves were constructed at the fixed-spot ablation and by the ablation under sample translation. The only disk for each percentage of cobalt was used for these experiments, because no statistically significant differences were found between parallel disks. The percentage values determined by XRF were employed for linear regression. 3.7. Calibration at fixed-spot ablation The fixed-spot ablation was repeated four times Ž ps 4. and six times Ž ps 6. at each percentage level to obtain two series of calibration data. The
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Table 3 Influence of the addition of hydrogen to plasma gas when spraying the WC᎐17%Co coating on the LA-ICP-AES signal ratio CorW a Flow rate of H2 Ždm3 miny1 .
CorW line intensity ratio
S.D.
%R.S.D.
3.0 8.0 13.0
0.347 0.329 0.301
0.005 0.008 0.006
1.3 2.4 1.9
a
S.D. stands for standard deviation. Number of disks for each H 2 flow rate was equal to 3.
number of percentage levels was equal to 4 Ž m s 4.. The sum of replicates of intensities of cobalt lines, ICo , which were obtained during the laser interaction time of 100 s, was taken as the analytical signal. Non-linear dependences of ICo on %Co were reproducibly observed for all employed cobalt lines. A linearization of these calibration curves was achieved using tungsten as internal standard. Average intensity ratio ICo rI W based on repetitions of ICo and I W was taken as the analytical signal. In this case, the first 30-s period of the laser interaction was taken as a preablation time and data acquired during the following 70-s period of ablation were considered for evaluation. A linear regression by the method of least squares was applied to compute the regression equation. Regression equations for four and six parallel measurements for Co 228 nm, Co 230 nm, Co 236 nm and Co 238 nm with W 224 nm as internal reference line are presented in Table 4. The upper and lower confidence limits c 2 c , c1 c about the centroid w21x of cobalt percentages are given by equations c 2 c ,c1 c s c c " t¨ ,0.05 sŽ y , x .rb w 1rqq 1r Ž mp .x
1r2
Ž 1,2. Here, c c is the centroid of cobalt percentages, t ,0.05 is the value of Student’s distribution function at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, sŽ y , x . is the standard deviation of points Žvalues of analytical signal. about the regression line, b is the slope of the regression line, q is a number of replicates of the intensity measurement on the percentage level c c , considered for the calculation of the confidence interval about the centroid. The widths of
these confidence intervals ⌬ c c s c 2 c y c1 c in Table 4 are lower for higher number of calibration points mps 24, as it might be expected. However, in spite of use of the background correction, non-zero intercepts on the intensity axis are statistically significant in all cases. Probably this is not only due to substantial extrapolation of the regression line down to zero percentage, representing 50% of the calibrated percentage range. Besides this extrapolation, the complex phase composition of WC-Co material w18᎐20x might be responsible for this fact. On the other hand, differences between slopes obtained for mps 16 and mps 24 are not statistically significant within a time period of 2 weeks ŽTable 4.. As an example, the calibration line ICo 228rI W 224 vs. %Co is presented in Fig. 4 Žcurve 1.. Ratio values are arithmetic means of 6 repetitions with %R.S.D. of single observation ranging from 1.8% to 2.5%. 3.8. Calibration at sample translation The ablation at sample translation was repeated three times Ž ps 3. and then four times Ž p s 4. at each percentage level. The laser interaction time was 10 min, which corresponded to 144 cycles. The translation velocity was 1 mm sy1 . Non-linear dependences of intensities on cobalt percentage were reproducibly observed for all employed cobalt lines. As an example, the dependence of ICo 228 vs. %Co is presented in Fig. 4 Žcurve 2.. The reason for this curvature consists probably in complex character of WC-Co coating. There exist large differences between temperatures of phase transformations of W2 C modifications, WC and other species. Within the carbon percentage
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Table 4 Statistical evaluation of calibration lines for Co in WC coatings a Co line Žnm.
r
⌬ cc Žwt.%.
100⌬ ccrcc Žrelative %.
b
sb
sb t0.05;
a
sa
ta
t0.05;
m=p
228 228 230 230 236 236 238 238
0.9950 0.9971 0.9948 0.9970 0.9954 0.9979 0.9907 0.9968
1.58 1.13 1.65 1.15 1.55 0.97 2.20 1.19
10.48 7.47 10.93 7.66 10.29 6.44 14.61 7.88
0.0175 0.0188 0.0164 0.0177 0.0578 0.0622 0.0995 0.1090
0.0005 0.0003 0.0005 0.0003 0.0015 0.0009 0.0036 0.0019
0.0010 0.0006 0.0010 0.0006 0.0032 0.0018 0.0078 0.0039
y0.0229 y0.0301 y0.0157 y0.0213 y0.1256 y0.1624 y0.1223 y0.1615
0.0075 0.0049 0.0072 0.0047 0.0240 0.0140 0.0586 0.0300
3.05 6.14 2.17 4.50 5.24 11.61 2.09 5.38
2.15 2.08 2.15 2.08 2.15 2.08 2.15 2.08
16 24 16 24 16 24 16 24
a Fixed-spot ablation for 100 s. Range of percentages 8᎐22.9% Co, centroid of percentages c c s 15.08% Co, confidence limits about the centroid c1 c , c 2 c , confidence interval about the centroid ⌬ c c s c2 c y c1 c , relative width of the confidence interval 100⌬ c crc c s 100Ž c 2 c y c1 c .rc c , number of calibration samples m s 4, number of parallel ablations on each calibration sample p s 4, and p s 6; number of replicates considered for the calculation of the confidence interval about the centroid q s 3, coefficient of correlation r, value of Student distribution function t ␣ ; at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, slope b, standard deviation of the slope sb , confidence limit about the slope sb t 0.05; , intercept on the intensity axis a, standard deviation of the intercept s a , calculated value of the Student test t a .
up to 6%, the eutectoidal transformation of the ␣-W2 C at 1300⬚C and solid᎐liquid equilibria of W2 C and WC at approximately 2700⬚C w17x occur. At a temperature of 1685⬚C, liquid Co and tungsten react to form an intermediate phase Co 7W6 which has a range of homogeneity between 70 and 76% W w18,19x. Moreover, numerous ternary species are formed in the system C᎐Co᎐W w20x. Besides this, the sample material is expected to be considerably changed in the ablation spot.
Calibration curves were linearised using internal standardization by tungsten. As an example, the calibration line ICo 228rI W 224 vs. %Co for mps 12 is presented in Fig. 4 Žcurve 3.. Regression equations using three and four parallel measurements Ž mps 12 and 16. for Co 228 nm, Co 230 nm, Co 236 nm and Co 238 nm analytical lines with W 224 nm as the internal reference line are presented in Table 5. For 50% of the regression lines the intercepts on the intensity axis are
Table 5 Statistical evaluation of calibration lines for Co in WC coatings a Co line Žnm.
r
⌬ cc Žwt.%.
100⌬ ccrcc Žrelative %.
b
sb
sb t0.05;
a
sa
ta
t0.05;
m=p
228 228 230 230 236 236 238 238
0.9976 0.9951 0.9978 0.9950 0.9966 0.9948 0.9962 0.9952
1.23 1.59 1.18 1.62 1.46 1.65 1.54 1.59
8.15 10.57 7.81 10.74 9.68 10.96 10.23 10.53
0.0174 0.0157 0.0164 0.0147 0.0582 0.0517 0.1004 0.0920
0.0004 0.0004 0.0004 0.0004 0.0015 0.0014 0.0028 0.0024
0.0009 0.0009 0.0008 0.0009 0.0034 0.0031 0.0062 0.0052
0.0009 0.0186 0.0066 0.0233 y0.0532 y0.0014 0.0125 0.0916
0.0062 0.0068 0.0056 0.0064 0.0246 0.0228 0.0449 0.0390
0.15 2.78 1.18 3.66 2.16 0.06 0.28 2.35
2.23 2.15 2.23 2.15 2.23 2.15 2.23 2.15
12 16 12 16 12 16 12 16
a Ablation for 10 min corresponds to 144 cycles, translation velocity was 1 mm sy1 . Range of percentages 8᎐22.9% Co, centroid of percentages c c s 15.08% Co, confidence limits about the centroid c1 c , c 2 c , confidence interval about the centroid ⌬ c c s c 2 c y c1 c , relative width of the confidence interval 100⌬ c crc c s 100Ž c2 c y c1 c .rc c , number of calibration samples m s 4, number of parallel ablations on each calibration sample ps 3, and ps 4; number of replicates considered for the calculation of the confidence interval about the centroid q s 3, coefficient of correlation r, value of Student distribution function t ␣ ; at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, slope b, standard deviation of the slope sb , confidence limit about the slope sb t 0.05; , intercept on the intensity axis a, standard deviation of the intercept s a , calculated value of the Student test t a .
V. Kanicky et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 575᎐586
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in Fig. 5. Dashed hyperbolae Žband 1. correspond to the regression uncertainty, dotted hyperbolae Žband 2. are the limits of the dispersion band. Individual observations of signal ratios ICo 228r I W 224 corresponding to calibration samples are presented, too. The parameters of this linear regression are given in the second row of the Table 5. 3.9. Analysis of unknown sample Fig. 4. Calibration curves: 1, ICo 228rI W 224 vs. %Co at the fixed spot ablation, mps 24; 2, ICo 228 vs. %Co at the ablation during sample translation; 3, ICo 228rI W 224 vs. %Co at the ablation during sample translation, mps 12. Error bars represent "standard deviation.
not statistically significant. Differences between slopes obtained for mps 12 and mps 16 are not statistically significant during the period of 2 weeks. Finally, two confidence bands along the regression line were computed w21,22x. The confidence band of the true regression line was computed for 95% probability according to the equations c 2 u ,c1 u s c u " Ž t¨ ,0.05 sŽ y , x .rb . 1r Ž mp . q Ž c u y c c . 2rÝ Ž c i y c c . 2
1r2
Ž 3,4.
where c u is the percentage of unknown sample, c 2 u , c1 u are corresponding confidence limits, c c is the centroid of percentages and c i is the ith percentage with i s 1 to mp. This band is related to the uncertainty due to the use of the calibration graph. The second confidence band, called the dispersion band, was computed for individual observation of analytical signal corresponding to c u , according to equations
The WC᎐Co top coating sprayed on 20 Nir80 Al intermediate layer was analyzed both using the WDXrEDX method and the LA-ICP-AES at sample translation. Conditions for laser ablation analysis were the same as for calibration study. The electron microprobe technique yielded the value of Ž11.9" 0.6.% Co Žfour replicates. and the LA-ICP gave the result Ž12.5" 0.8.% Co based on three replicates. The ‘"’ values denotes the confidence interval limits.
4. Conclusions The results presented in this work suggest that laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES. could be used for characterisation of tungsten carbide coatings containing cobalt. Homogeneity of plasma sprayed coatings was tested with the
c 2 u ,c1 u s c u " Ž t¨ ,0.05 sŽ y , x .rb . 1 q 1r Ž mp . q Ž c u y c c . 2rÝ Ž c i y c c . 2
1r2
Ž 5,6.
Both the confidence bands are drawn along the calibration line ICo 228rI W 224 vs. %Co for mps 12
Fig. 5. Calibration curve ICo 228rI W 224 vs. %Co obtained at sample translation, mps 12. Confidence bands: 1, regression band; 2, dispersion band.
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resulting relative standard deviation of the ratio CorW approximately 4%. The LA-ICP-AES signal ratio of CorW was found to be sensitive to variation of the gas mixture composition ŽH 2rAr. at sample spraying because the intensity ratio changes were statistically significant. Time correlation of signals of W and Co was proved to be useful for linearization of calibration curves of cobalt. Due to the relative width of the confidence interval about the centroid of calibration lines, which is approximately "5%, the determination of Co can be regarded rather semiquantitative. However, the X-ray-based techniques ŽXRF, EDXrWDX. yielded results with comparable precision. Improvements of precision could be expected for ablation of greater sample surface. This is the aim of our further studies.
Acknowledgements V.K. and V.O. gratefully acknowledge the Grant Agency of the Czech Republic for bestowing the financial support Žproject 203r97r0345. and thank the Ministry of Education and Youth of the Czech Republic for the grant to the project VS 97020. V.K. and V.O. thank J. Filipensky ´ and J. Ondracek ´ˇ from Plasmametal Co., Brno, Czech Republic and O. Ambroz ˇ from the Faculty of Mechanical Engineering of the Technical University Brno, Czech Republic, for the preparation of plasma-sprayed coatings. V.K. and V.O. thank P. Janosˇ and V. ˇ Sprta from the Research Institute of ´ ´ nad Labem, Czech ReInorganic Chemistry, Ustı public, for the XRF analyses of samples and P. Sulovsky ´ from the Department of Mineralogy, Petrology and Geochemistry of the Faculty of Science of the Masaryk University Brno, Czech Republic, for the EDXrWDX analyses of samples. The authors wish to thank Perkin-Elmer for the loan of the Optima 3000 DV ICP spectrometer.
References w1x M.M. Schwartz ŽEd.., Handbook of Structural Ceramics, Mc Graw-Hill, New York, 1992. w2x V. Kanicky, ´ I. Novotny, ´ J.L. Lacour, P. Mauchien, J.M. Mermet, J. Phys. 4 Ž1994. 710. w3x V. Kanicky, ´ J. Musil, J.M. Mermet, Appl. Spectrosc. 51 Ž1997. 1037. w4x V. Kanicky, ´ J. Musil, M. Benda, J.M. Mermet, Collect. Czech. Chem. Commun. 61 Ž1996. 1167. w5x V. Kanicky, ´ I. Novotny, ´ J. Musil, J.M. Mermet, Appl. Spectrosc. 51 Ž1997. 1041. w6x D. Potter, I. Abell, Anal. Sci. 7 Ž1991. 1239. w7x J.S. Becker, U. Breuer, J. Westheide, A.I. Saprykin, H. Holzbrecher, H. Nickel, H.J. Dietze, Fresenius’ J. Anal. Chem. 355 Ž1996. 626. w8x J. Musil, J. Therm. Spray Technol. 6 Ž1997. 387. w9x J. Musil, J. Fiala, Surface Coating Technol. 52 Ž1992. 211. w10x J. Musil, M. Alaya, R. Oberacker, J. Therm. Spray Technol. 6 Ž1997. 449. w11x K.A. Khor, P. Cheang, C.S. Yip, J. Therm. Spray Technol. 6 Ž1997. 109. w12x P. Filip, K. Mazanec, A.C. Kneissl, R. Melicharek, Z. ´ Metallkd. 88 Ž1997. 131. w13x Hardware Guide OPTIMA 3000, Ch. 2, Perkin Elmer Corp., Norwalk, CT, 1993, p. 5. w14x M. Hemmerlin, J.M. Mermet, M. Bertucci, P. Zydowicz, Spectrochim. Acta Part B 52 Ž1997. 421. w15x H. Remy, Lehrbuch der Analytischen Chemie, Band II, Akademische Verlagsgesellschaft Geest & Portig KG, Leipzig, 1961. w16x F.A. Cotton, G.W. Wilkinson, Advanced Inorganic Chemistry, John Wiley & Sons, New York, 1966. w17x G.V. Raynor, V.G. Rivlin, Phase Equilibria in Iron Ternary Alloys, The Institute of Metals, 1988. w18x J. Kubıcek, ´ˇ L. Ivan, Proceedings of the 1st Czech National Thermal Spray Conference, Brno, Czech Republic, 19᎐24 April 1994, p. 111. ˇ w19x M. Cepera, J. Zeman, J. Musil, P. Fiala, J. Filipensky, ´ Proceedings of the 1st Czech National Thermal Spray Conference, Brno, Czech Republic, 19᎐24 April 1994, p. 115. w20x G. Barbezat, E. Muller, B. Walser, Sulzer Tech. Rev. 4 ¨ Ž1988. 4. w21x J.M. Mermet, Spectrochim. Acta Part B 49 Ž1994. 1313. w22x D.L. Massart, B.G.M. Vandeginste, S.N. Deming, Y. Michotte, L. Kaufman, Chemometrics, a Textbook, Elsevier, Amsterdam, 1988.
Analysis of tungsten carbide coatings by UV laser ablation inductively coupled plasma atomic emission spectrometry V. Kanicky a,U , V. Otrubaa , J.-M. Mermet b a
Laboratory of Plasma Sources for Chemical Analysis, Faculty of Sciences, Masaryk Uni¨ ersity Brno, Kotlarska 2, CZ 61137 Brno, Czech Republic b Laboratoire des Sciences Analytiques (CNRS UMR 5619), Uni¨ ersite´ Claude Bernard-Lyon I, Batiment 308, F 69622 Villeurbanne, France Received 3 December 1999; accepted 15 February 2000
Abstract Tungsten carbide coatings Žthickness 0.1᎐0.2 mm. containing 8.0, 12.2, 17.2 and 22.9% Co were studied with laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES.. Composition of these plasma sprayed deposits on steel disks was determined using X-ray fluorescence spectrometry and electron microprobe energyrwavelength dispersive X-ray spectrometry. The coatings were ablated by means of a Q-switched Nd:YAG laser at 266 nm Ž10 Hz, 10 mJ per shot. coupled to an ICP echelle-based spectrometer equipped with a segmented charge-coupled device detector. Non-linear dependences of cobalt lines intensities on the Co percentage were observed both at a single spot ablation and at a sample translation. This behaviour could be attributed to a complex phase composition of the system W᎐C᎐Co. However, employing tungsten as internal standard the linear calibration was obtained for studied analytical lines Co II 228.616 nm, Co II 230.786 nm, Co II 236.379 nm and Co II 238.892 nm. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: UV laser ablation; Tungsten carbide coatings; Inductively coupled plasma; Atomic emission spectrometry
U
Corresponding author. Tel.: q42-5-41129283; fax: q42-5-41211214. E-mail address: [email protected] ŽV. Kanicky.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 1 6 7 - 1
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1. Introduction Over two past decades, laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES. has become an efficient tool for direct analysis of compact solids, as a laser beam allows vaporisation of chemically resistant materials, both conducting and non-conducting. Since the mid-1960s, composite materials, or structural ceramics, have been gaining increased importance in various production technologies. Advanced ceramics are now finding widespread use in many load-bearing applications, such as cutting tools, car-engine components, heat exchangers, turbines, wear parts, and aerospace, where they are applied as protective coatings w1x. Determination of chemical composition of these coatings might be a particular task for the laser ablation-based spectrometry. Recently, LA-ICP-AES was applied to the determination of Zr and Ti in 3-m-thick ZrTiN coatings, which were prepared by physical vapour deposition process on a high-speed steel substrate w2,3x. Nitrided steel surfaces with 5-m layers of sputtered Ti or V were investigated using frequency-tripled and frequency-quadrupled Qswitched Nd:YAG laser coupled to an ICP-AES w4x. Qualitative depth profiles of zirconia graded metal-ceramic coatings of thickness of 700 m, prepared by plasma-spraying on steel substrate, were obtained by means of a frequency-quadrupled, pulsed Nd:YAG laser with an ICP emission spectrometer w5x. Some high-tech ceramic materials, such as ceramic materials for electronic w6x, and ceramic layers of solid oxide fuel cells w7x were analyzed using LA with ICP mass spectrometry ŽICP-MS.. Thermal spraying of ceramic coatings represents very useful way of surface treatment of metal parts and tools that are exposed to extreme mechanical loading, thermal shocks andror chemical attacks w8x. Information on homogeneity and chemical composition of these ceramic layers is important for optimization of production technology. Being highly chemically resistant, these coatings are usually examined by means of techniques of surface analysis, such as glow discharge optical emission spectrometry, secondary ion mass
spectrometry, X-ray fluorescence spectrometry, and electron microprobe energy dispersive X-ray spectrometry. However, these techniques allow to obtain chemical information restricted only to near-surface layers, while the thickness of the coatings such as partially stabilized ZrO 2 thermal barrier coatings w9,10x or hydroxyapatite composite coatings for bioceramics w11,12x is approximately several hundreds of micrometers. In such cases, average in-depth composition could be possibly determined by means of LA-ICP spectrometry. The aim of this work was to study the possibility of using LA-ICP-AES ablation for the determination of cobalt content in tungsten carbide ŽWC-Co. coatings, which were sprayed on steel substrates by means of a microprocessor-controlled plasmatron fed with WC powders containing different percentages of Co. The critical parameter was expected to be the homogeneity of coatings and different physical characteristics of Co and tungsten carbide. Therefore, attention was paid to evolution of temporal LA-ICP-AES signals, correlation of Co and W line intensities and internal standardization using tungsten lines. Reproducibility of ablation, linearity of calibration, and precision and accuracy of determination were investigated using tungsten carbide samples with four different percentages of cobalt.
2. Experimental 2.1. Instruments and operating conditions A Q-switched Nd:YAG laser Žmodel Surelite Ir20, Continuum, USA. was used for ablation at its 4th harmonics Ž266 nm. with the laser pulse width of 5 ns and the laser pulse repetition rate of 10 Hz. The pulse energy of 10 mJ was adjusted by setting the Xe-flashlamp discharge voltage. The beam expander ŽModel BXG0-10.0-3X-266, CVI Laser Corporation, USA. was located between the laser and the focusing plano-convex quartz lens with a 270-mm focal length in the visible range. The beam expander output lens was 150 mm in front of this lens. The optimum focusing was achieved when observing the highest acoustic
V. Kanicky et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 575᎐586
emission during the ablation process and the resulting laser spot diameter was approximately 0.1 mm. The average power was measured with an Astral TM Laser PowerrEnergy Meter, Model AD 30 ŽScientech, USA.. The ablation cell consisted of a sample holder and a glass chamber of the volume of 140 cm3, provided with a silica window. The carrier gas Ž0.65 dm3 miny1 Ar. was introduced through a gas inlet into the chamber in the parallel direction to the sample surface. The gas outlet was situated opposite to the inlet. The ablated material was transported by the carrier argon flow along a polyamide tubing Žlength 1.5 m, i.d. 4 mm. through the standard double-pass Scott spray chamber to the ICP. The cell was mounted on a computer controlled, motor-driven XY-translation stage ŽModel PCC 2r4, Schneeberger, France. to be moved with respect to the laser beam which was perpendicular to the target surface. Both fixed-spot ablation and ablation with translation along the 1 mm= 1 mm-square pattern were carried out. The translation speed was always 1 mm sy1 . A Perkin-Elmer Optima 3000 ICP Dual View system was used in the axial observation mode for the analytical signal measurements. The 40-MHz free-running generator was operated at the forward power of 1.1 kW, and the outer, intermediate and carrier gas flow rates were 15.0, 0.5 and 0.65 dm3 miny1 of argon, respectively. The carrier gas flow rate was optimized to obtain maximum signal-to-background ratio. The flat maximum between 0.6 and 0.7 dm3 miny1 was found. The polychromator with an echelle grating was provided with segmented-array charge-coupled device detectors ŽSCD.. The spectral bandwidth of the polychromator is 0.007 nm at 220᎐240 nm w13x. This system allowed simultaneous measurement of several spectral lines including their off-peak background correction positions. The sum of three adjacent pixels of the SCD was taken to measure the gross emission intensity Ža peak area mode.. This total intensity was corrected for the off-peak background emission. The sampling time of 1 s and integration times in the 5᎐200-ms range depending on the line intensity were employed for all measurements. On average, a signal was available every 2.4 s, including the
577
sampling time and the detector reading and transfer time. The following analytical lines were measured Žwavelength in nm.: Co II 228.616; Co II 230.786; Co II 236.379; Co II 238.892; W II 207.911; W II 224.875; W II 248.924; Fe II 238.204 and C I 193.018. Tungsten carbide represents a difficult sample matrix from the viewpoint of both the possible spectral and non-spectral interferences. A great attention was paid to the selection of background correction wavelength positions. The following possible mutual spectral influences were expected: Co II 228.616 nm could be influenced by W 228.629 nm and W 228.590 nm; Co II 230.786 nm by W II 230.793 nm and W 230.766 nm; Co II 236.379 nm by W 236.389 nm; Co II 238.892 by W 238.880 nm and W 238.907; and vice versa, W II 207.911 nm could be influenced by Co I 207.931 nm; W II 224.875 nm by Co II 224.866 nm; W II 248.923 nm by Co II 248.925 nm. Overcorrection was observed for Co II 236.379 nm due to the interference of W II 236.389 nm on the background correction position at the longer wavelength, and therefore, only correction position at the shorter wavelength was used. Spectral interference of Fe appeared after penetrating of the laser beam into the steel substrate. The short-wavelength background correction position at 238.870 nm of the Co II 238.892 nm was overlapped by Fe II 238.868 nm. Therefore, only long-wavelength background position Ž238.921 nm. was used for correction. In other cases no spectral interferences were observed. 2.2. Samples Samples of tungsten carbide coatings containing cobalt were prepared by means of a commercial microprocessor-controlled atmospheric plasma spraying system ŽSulzer Plasma-Technik AG, Switzerland. in the facility of Plasmametal, Co. Ltd., Brno, Czech Republic. Following deposition parameters were employed: electrical power 28 kW, discharge current 600 A, plasma gas flow rate 42 dm3 miny1 of Ar, spraying distance 130 mm. Hydrogen Ž3 dm3 miny1 . was being added to the plasma argon flow during the spraying. Four powdered samples of tungsten carbide with
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declared percentages of Co Ž6%, 12%, 17% and 20%. were plasma-sprayed on steel disks with diameter of 58 mm and thickness of 1.5 mm. To find the exact value of the coating thickness, the sample was cut using the diamond circular saw ŽDepartment of Mineralogy, Petrology and Geochemistry, Masaryk University Brno. and the width of the coating was measured with Abbe comparator Žmade by Carl Zeiss Jena, Germany.. The thickness of coatings was in the range 0.10᎐0.17 mm. At each percentage of cobalt, three coated disks were prepared under identical conditions with the aim of studying the sample preparation reproducibility. The influence of the hydrogen flow rate was studied on samples containing 17% of Co. For this purpose, three disks were prepared for each of additional flow rates: 8 dm3 miny1 of H 2 and 13 dm3 miny1 of H 2 . Basic characteristics of WC᎐Co powders and thickness of coatings are given in Table 1a. A sample prepared independently in another laboratory ŽFaculty of Mechanical Engineering, Technical University Brno, Czech Republic. was used as an unknown sample for analysis. This sample consisted of WCq 12% Co top coating, which was plasma-sprayed on intermediate binding coat Ž20% Nir80% Al.. The total thickness of the combined deposit was 0.24 mm. Chemical composition of plasma sprayed coatings was determined by wavelength-dispersive X-ray fluorescence spectrometry ŽXRF. and electron microprobe-energyrwavelength-dispersive X-ray spectrometry ŽEDXrWDX.. In order to match the sample size to dimensions of the XRF and EDXrWDX sample compartments, one disk for each Co percentage value was cut into four parts that were embedded with polymethyl-
metacrylate ŽPMM.. Four disks of the diameter of 40 mm were obtained in this way. The samples were cleaned after cutting in the ultrasonic bath using ethanol. The XRF and EDXrWDX analyses were carried out at each of the four PMMembedded samples for all Co percentages. The XRF analysis was performed using the sequential spectrometer PW 1404 ŽPhilips, the Netherlands. equipped with a rhodium target X-ray tube and the LiF 220, Ge111 and TlAP analyzer crystals. Percentages were calculated by means of a UNIQUANT software ŽPhilips., a precalibrated analytical program for semiquantitative analysis of unknown samples. UNIQUANT determines the net intensities of fluorescence lines and corrects for matrix interelement effects using ␣-coefficients computed with the method of fundamental parameters. The samples were analyzed at the Research Institute of Inorganic Chemistry, ´ stı´ nad Labem, Czech Republic. U The EDXrWDX analysis was performed with the electron scanning microscope system CamScan 4DD ŽCambridge Instruments, UK. coupled to the energy-dispersive X-ray analyzer LINK AN 10000 and wavelength-dispersive X-ray analyzer MICROSPEC 2A. Analyzers were calibrated with 100% W, 100% C and 100% Co. The results were normalised to the sum of 100%. These analyses were carried out at the Department of Mineralogy, Petrology and Geochemistry, Masaryk University Brno, Czech Republic. Results of XRF and EDXrWDX determinations are presented in Table 1b. Confidence intervals st n,1y␣ of the mean values of percentages were calculated based on four determinations. Iron was detected as the main impurity in all samples. It follows from presented data that no
Table 1a Description of the tungsten carbidercobalt powders and thickness of plasma sprayed coatings Material
Declared powder composition
Grain size Žm.
Coating thickness Žmm.
Metco Metco 71 VF NS PT 7446 PT T 5006
WCq 6% Co WCq 12% Co WCq 17% Co WCq 20% Co
y45 q 5 y45 q 5 y53 q 5 y45 q 11
0.12 0.12 0.10 0.17
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Table 1b Chemical composition of the plasma sprayed coatings determined with wavelength dispersive X-ray fluorescence spectrometry ŽXRF. and electron microprobe-energyrwavelength-dispersive X-ray spectrometry ŽEDXrWDX. a Material
Metco Metco 71 VF NS PT 7446 PT T 5006
XRF
EDXrWDX
Co Žwt.%.
W Žwt.%.
c
stn,1y␣
c
stn,1y␣
8.0 12.2 17.2 22.9
0.2 0.3 0.3 0.3
82.0 81.0 76.8 70.6
0.8 0.6 0.6 0.6
Fe Žwt.%. c
Co Žwt.%.
W Žwt.%.
c
stn,1y␣
c
stn,1y␣
3.5 1.9 0.9 1.0
7.8 11.5 17.1 22.6
0.3 0.4 0.6 1.2
79.3 78.8 77.5 72.0
0.7 1.5 1.6 1.0
Fe Žwt.%. c 4.2 1.5 0.6 0.9
a Confidence intervals st n,1y␣ are calculated at the level of significance ␣ s 0.05 for number of repetitions n s 4; wt.% means weight percentage.
statistically significant differences exist between results of both methods.
3. Results and discussion 3.1. Temporal beha¨ iour of signals Fig. 1 presents the temporal behaviour of signals of Co, W and C during ablation of the WC sample containing 8% Co ŽTable 1b.. Under translation, the intensities of W II 224.875 nm, Co II 228.616 nm and C I 193.018 nm lines reached the plateau after approximately 120 s and remained practically constant at least for 8 min Žcurves 1, 2 and 3 in Fig. 1.. Identical curves were obtained for all other cobalt and tungsten lines. Simultaneously, iron impurities within the coating were detected Žcurve 4, Fig. 1. and thus the results obtained by both X-ray methods were qualitatively proved. At the fixed-spot ablation, all the signals attained lower values in comparison to the translation experiment and the rapid decrease of intensities with time was observed after reaching their maxima Žcurves 5, 6 and 7 in Fig. 1.. This corresponds well to observations already described, e.g. at the ablation of partially stabilized zirconia coatings w5x and it could be possibly explained by hindered releasing of vaporized material from the single pit. Evolution of temporal signals during the fixed-spot ablation was less reproducible than under translation of sample, which might be due to local inhomogeneities. At the fixed-spot abla-
tion, the coatingrsubstrate interface was reached after approximately 2 min which implied from the steep increase of the Fe II 238.204 nm line intensity Žcurve 8, Fig. 1.. The non-zero signal of the iron line, which occurred before reaching the interface, confirmed the presence of Fe in the substrate. However, the increasing parts of all the temporal signals did not represent the true time courses of the laser᎐coating or laser᎐interface interactions, since the large dead volume of the ablation cell contributed to the rise-time of temporal signals. Besides this, the ablation of the coating continued even after penetrating of the beam into the substrate, because of conical shape of the crater.
Fig. 1. Laser ablation-ICP-AES temporal signals at the sample translation Žcurve 1, Co II 228.616 nm; 2, W II 224.875 nm; 3, C I 193.018 nm; 4, Fe II 238.868 nm. and at the fixed-spot ablation Žcurve 5, Co II 228.616 nm; 6, W II 224.875 nm; 7, C I 193.018 nm; 8, Fe II 238.868 nm..
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Although the signal of carbon exhibited a magnitude far above the background level, no analytical use was made of it because of a high ‘blank value’ and a monotonous content of carbon Žapprox. 5% C. in our samples. The curves 3 and 7 in Fig. 1 represent signals of the C 193.018 nm line, corrected both for the spectral background and the line intensity resulting from the presence of CO 2 in the ambient air. 3.2. Internal standardization and repeatability The possibility to use a major constituent of a sample matrix as an internal standard is of a great significance for improving precision, repeatability, reproducibility and accuracy of laser ablation analysis. The terms of repeatability and reproducibility used here are consistent with those introduced in w14x. In that paper the repeatability was estimated by dividing a temporal ablation signal, corresponding to one line pattern ablated in a sample, into 10 parts and calculating the relative standard deviation between the 10 mean values obtained w14x. In case of a tungsten carbidercobalt system, a complex behaviour at a laser᎐material interaction could be expected due existing numerous phases of different properties and composition, such as WC, W2 C, WC 1yx , Co 3 C, Co 2 C, Co 7W6 , Co 3W3 C, Co 3W9 C, Co 3W9 C 4 , Co 6W6 C w15᎐20x. Moreover, Co and W themselves differ substantially in melting Žm.t.. and boiling Žb.t.. temperatures: m.t.s 1490⬚C and 3400⬚C, b.t.s 3100⬚C and 5700⬚C, for Co and W, respectively w15x. For evaluating the possibility of internal standardization by tungsten, the ratios of the temporal signals presented in Fig. 1 were plotted in Fig. 2. At first, the signals obtained by ablation of a translated sample are discussed. It is obvious from the curve 1 in Fig. 2, that the intensity ratio of Co II 228.616 nm and W II 224.875 nm lines remained constant over the ablation period of 500 s after the rise-time Ž100 s.. Furthermore, the fluctuations of this ratio were considerably smaller in comparison to individual line intensities, see curves 1 and 2 in Fig. 1. The coefficient of correlation r between 83 data points for Co II 228.616 nm and W II 224.875 nm lines over the time
Fig. 2. Temporal behaviour of LA-ICP-AES intensity ratios at the sample translation Žcurves 1, CorW; 2, CrW. and at the fixed-spot ablation Žcurves 3, CorW; 4, CrW.. Employed lines: Co II 228.616 nm, W II 224.875 nm, C I 193.018 nm.
period from 200 s to 400 s was equal to 0.9792. Consequently, the relative standard deviation Ž%R.S.D.. of the signal ratios of Co and W within this period was 0.60%, while the %R.S.D. of the Co signals without internal standardization to W was 2.65%. This means that using internal standardization the signal stability was increased, or in other words, repeatability as defined in w14x was improved by a factor of 4.4. Surprisingly, a slight improvement by internal standardization was observed even for C I 193.018 nm line. The rising part of the curve 3 in Fig. 1 was straightened, as it is obvious from curve 2 in Fig. 2. However, the moderate increase of the carbon signal after 300 s of ablation was not entirely eliminated, compare with curve 3 in Fig. 1. Within the same period as for cobalt, the coefficient of correlation between C I 193.018 nm and W II 224.875 nm line intensities reached only the value 0.5952. Accordingly, the %R.S.D. of the C IrW II intensity ratio remained the same as the %R.S.D. of the carbon signal itself, i.e. 2.8%, and no improvement of repeatability was observed. It can be supposed that the correlation carbon᎐tungsten would be better if the blank signal of carbon were substantially lower than the signal originating in the sample. A more striking effect was observed at the fixed-spot ablation. In this case, the steep drop of both the Co II 228.616 nm and C I 193.018 nm
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line intensities Žcurves 5 and 7 in Fig. 1. was compensated by W II 224.875 nm line intensity, see curves 3 and 4 in Fig. 2. This straightening of curves continued even beyond the coatingrsteel interface. All the described features of LA-ICP-AES signals were found for other studied spectral lines of W and Co, for other WC᎐Co coatings and for each concentration level of cobalt. 3.3. Influence of crater depth on magnitude of LAICP-AES signals and estimation of ablation rate The diminution of the LA-ICP signals as a consequence of increasing depth of the ablation crater was demonstrated also by the behaviour of the signal of Fe originating from the coatingrsubstrate interface. The fixed-spot ablation was performed on samples with different coating thickness ŽTable 1a. and the signal of Fe was measured after reaching the interface. The decrease of the Fe II 238.204 nm line intensity with the increasing thickness of the WC᎐Co coating is plotted in Fig. 3, see curve 1. The error bars represent standard deviations based on four replicates. As the composition of the steel disks was identical, the lowering of the signal of Fe was due to a restraint caused by the narrow and deep crater to the release and transport of ablated material from the ablation spot. These obstructions to the aerodynamic flow have been already described when a depth-profiling of thick zirconia ceramic coatings was performed w5x. The time elapsed from the beginning of ablation to reaching the steel substrate was measured and plotted against the coating thickness Žcurve 2 in Fig. 3.. The linear regression yielded the equation, Time Žs. s 1173 = Thickness Žmm., with the coefficient of correlation r s 0.9805. The intercept was statistically insignificant. The ablation rate averaged over the coating depth in terms of the linear velocity of penetration was therefore constant within the experimentally studied thickness up to 0.17 mm, within the experimental errors and for four different coating compositions. This average ablation rate was equal to 0.87 m sy1 , or 0.087 m per shot.
Fig. 3. Influence of crater depth on magnitude of LA-ICP-AES signals and estimation of ablation rate. Dependence of the Fe II 238.868 nm line intensity originating from the ablation of the steel substrate on the thickness of the WC᎐Co coating at the fixed-spot ablation Žcurve 1.. Dependence of the time of penetration of the laser beam to the coatingrsubstrate interface on the coating thickness Žcurve 2. for the estimation of the ablation rate.
3.4. Reproducibility of ablation and homogeneity of samples Due to the technology of preparation, the WC᎐Co coatings were expected to be inhomogeneous to some extent, and therefore, inappropriate for microsampling by a laser beam. To find possible sources of uncertainties and to evaluate limitations for quantitative analysis, the ablation measurements were performed on all disks for a given percentage of cobalt and for each percentage level. The fixed-spot ablation was repeated five times on each disk. The laser interaction time was always 100 s to avoid the ablation of the coatingrsubstrate interface. Owing to the shapes of the fixed-spot temporal signals ŽFig. 1, curves 5, 6. the sum of replicates of ICP measurements that were obtained during the interaction time was taken as the analytical signal. Consequently, this analytical signal represented the average in-depth composition. Reproducibility of the analytical signal for a single disk was evaluated by calculating the %R.S.D. between the analytical signals recorded for five individual ablation pits in one disk, i.e. as already defined in w14x. Reproducibility of the analytical signal for the set of disks was evaluated by calculating the %R.S.D. between the analytical signals recorded for three
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ablation pits, each obtained on individual disk. This %R.S.D. for both the Co and W analytical signals was in the range from 7 to 15% regardless of the fact whether it was calculated within a single disk or between disks. Such errors seemed to be unacceptable for further evaluation and for that reason the CorW signal ratio was only used for following tests. The data for four spectral lines of Co and the W II 224.875 nm line as the internal standard were tested using the analysis of variance ŽANOVA.. From the ANOVA results it followed that the spread of CorW signal ratios within the disks did not differ significantly from the spread between the disks. As an example, the ANOVA results for the Co II 228.616 nm line are presented in Table 2. It is evident that calculated F-values are lower than the critical value Fcrit Ž 1 , 2 . of the Fisher᎐Snedecor distribution. The %R.S.D. values of the reproducibility of CorW signal ratios were within the range from 1.5 to 4% for all percentages of Co and also for all Co lines. Therefore, it could be concluded that the reproducibility within a single sample as well as between the samples of the same composition or even between different percentage levels is acceptable for quantitative analysis, with the relative standard deviation better than 4%. As we had no alternative WC᎐Co samples prepared with different technology providing high degree of homogeneity, we could not evaluate the reproducibility
of the ablation itself. Consequently, we can only conclude that the homogeneity of the plasma sprayed coatings corresponds with value of R.S.D. lower than 4%. 3.5. Influence of preparation conditions of coatings on analytical signal By the plasma spraying of the cermet coatings, hydrogen is usually added to the plasma gas flow of argon for increasing the heat transfer to particles of sprayed materials. The proportion of H 2 to Ar influences also the focusing of separate impingement particle spots on the substrate w9x. This could affect the properties of the coating. The influence of the H 2 flow rate on the LA-ICPAES signal ratio of CorW was investigated at three H 2 flow rates: 3, 8 and 13 dm3 miny1 . For each H 2 flow rate, three sprayed disks ŽWCr17% Co. were prepared. Fixed-spot ablation was performed on each disk and the data were evaluated using ANOVA. Average values and relative standard deviations calculated based on three disks for each level of hydrogen flow rate are given in Table 3. The decrease of LA-ICP-AES intensity ratio of CorW with increasing H 2 flow rate is statistically significant, because calculated F-value is equal to 17.35, and therefore substantially greater than the critical value Fcrit Ž2, 6. s 5.14 of the Fisher᎐Snedecor distribution. 3.6. Study of calibration
Table 2 Results of the analysis of variance for evaluation of the reproducibility of ablation a %Co
F
%R.S.D.
8.0 12.2 17.2 22.9
1.03 0.28 3.55 1.77
3.2 3.8 1.7 3.1
a
Evaluated signals: intensity ratios Co 228.616 nmrW 224.875 nm. Number of repetitions per disk, ps 5; number of disks per percentage of cobalt, n s 3; the critical value Fcrit Ž 1 , 2 . of the Fisher᎐Snedecor distribution for 1 s n y 1, 2 s n = py n, Fcrit Ž2, 12. s 3.885, the calculated value of F, the relative standard deviation %R.S.D. of a single observation calculated using n = p observations, i.e. the %R.S.D. of the reproducibility of ablation.
Calibration curves were constructed at the fixed-spot ablation and by the ablation under sample translation. The only disk for each percentage of cobalt was used for these experiments, because no statistically significant differences were found between parallel disks. The percentage values determined by XRF were employed for linear regression. 3.7. Calibration at fixed-spot ablation The fixed-spot ablation was repeated four times Ž ps 4. and six times Ž ps 6. at each percentage level to obtain two series of calibration data. The
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Table 3 Influence of the addition of hydrogen to plasma gas when spraying the WC᎐17%Co coating on the LA-ICP-AES signal ratio CorW a Flow rate of H2 Ždm3 miny1 .
CorW line intensity ratio
S.D.
%R.S.D.
3.0 8.0 13.0
0.347 0.329 0.301
0.005 0.008 0.006
1.3 2.4 1.9
a
S.D. stands for standard deviation. Number of disks for each H 2 flow rate was equal to 3.
number of percentage levels was equal to 4 Ž m s 4.. The sum of replicates of intensities of cobalt lines, ICo , which were obtained during the laser interaction time of 100 s, was taken as the analytical signal. Non-linear dependences of ICo on %Co were reproducibly observed for all employed cobalt lines. A linearization of these calibration curves was achieved using tungsten as internal standard. Average intensity ratio ICo rI W based on repetitions of ICo and I W was taken as the analytical signal. In this case, the first 30-s period of the laser interaction was taken as a preablation time and data acquired during the following 70-s period of ablation were considered for evaluation. A linear regression by the method of least squares was applied to compute the regression equation. Regression equations for four and six parallel measurements for Co 228 nm, Co 230 nm, Co 236 nm and Co 238 nm with W 224 nm as internal reference line are presented in Table 4. The upper and lower confidence limits c 2 c , c1 c about the centroid w21x of cobalt percentages are given by equations c 2 c ,c1 c s c c " t¨ ,0.05 sŽ y , x .rb w 1rqq 1r Ž mp .x
1r2
Ž 1,2. Here, c c is the centroid of cobalt percentages, t ,0.05 is the value of Student’s distribution function at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, sŽ y , x . is the standard deviation of points Žvalues of analytical signal. about the regression line, b is the slope of the regression line, q is a number of replicates of the intensity measurement on the percentage level c c , considered for the calculation of the confidence interval about the centroid. The widths of
these confidence intervals ⌬ c c s c 2 c y c1 c in Table 4 are lower for higher number of calibration points mps 24, as it might be expected. However, in spite of use of the background correction, non-zero intercepts on the intensity axis are statistically significant in all cases. Probably this is not only due to substantial extrapolation of the regression line down to zero percentage, representing 50% of the calibrated percentage range. Besides this extrapolation, the complex phase composition of WC-Co material w18᎐20x might be responsible for this fact. On the other hand, differences between slopes obtained for mps 16 and mps 24 are not statistically significant within a time period of 2 weeks ŽTable 4.. As an example, the calibration line ICo 228rI W 224 vs. %Co is presented in Fig. 4 Žcurve 1.. Ratio values are arithmetic means of 6 repetitions with %R.S.D. of single observation ranging from 1.8% to 2.5%. 3.8. Calibration at sample translation The ablation at sample translation was repeated three times Ž ps 3. and then four times Ž p s 4. at each percentage level. The laser interaction time was 10 min, which corresponded to 144 cycles. The translation velocity was 1 mm sy1 . Non-linear dependences of intensities on cobalt percentage were reproducibly observed for all employed cobalt lines. As an example, the dependence of ICo 228 vs. %Co is presented in Fig. 4 Žcurve 2.. The reason for this curvature consists probably in complex character of WC-Co coating. There exist large differences between temperatures of phase transformations of W2 C modifications, WC and other species. Within the carbon percentage
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Table 4 Statistical evaluation of calibration lines for Co in WC coatings a Co line Žnm.
r
⌬ cc Žwt.%.
100⌬ ccrcc Žrelative %.
b
sb
sb t0.05;
a
sa
ta
t0.05;
m=p
228 228 230 230 236 236 238 238
0.9950 0.9971 0.9948 0.9970 0.9954 0.9979 0.9907 0.9968
1.58 1.13 1.65 1.15 1.55 0.97 2.20 1.19
10.48 7.47 10.93 7.66 10.29 6.44 14.61 7.88
0.0175 0.0188 0.0164 0.0177 0.0578 0.0622 0.0995 0.1090
0.0005 0.0003 0.0005 0.0003 0.0015 0.0009 0.0036 0.0019
0.0010 0.0006 0.0010 0.0006 0.0032 0.0018 0.0078 0.0039
y0.0229 y0.0301 y0.0157 y0.0213 y0.1256 y0.1624 y0.1223 y0.1615
0.0075 0.0049 0.0072 0.0047 0.0240 0.0140 0.0586 0.0300
3.05 6.14 2.17 4.50 5.24 11.61 2.09 5.38
2.15 2.08 2.15 2.08 2.15 2.08 2.15 2.08
16 24 16 24 16 24 16 24
a Fixed-spot ablation for 100 s. Range of percentages 8᎐22.9% Co, centroid of percentages c c s 15.08% Co, confidence limits about the centroid c1 c , c 2 c , confidence interval about the centroid ⌬ c c s c2 c y c1 c , relative width of the confidence interval 100⌬ c crc c s 100Ž c 2 c y c1 c .rc c , number of calibration samples m s 4, number of parallel ablations on each calibration sample p s 4, and p s 6; number of replicates considered for the calculation of the confidence interval about the centroid q s 3, coefficient of correlation r, value of Student distribution function t ␣ ; at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, slope b, standard deviation of the slope sb , confidence limit about the slope sb t 0.05; , intercept on the intensity axis a, standard deviation of the intercept s a , calculated value of the Student test t a .
up to 6%, the eutectoidal transformation of the ␣-W2 C at 1300⬚C and solid᎐liquid equilibria of W2 C and WC at approximately 2700⬚C w17x occur. At a temperature of 1685⬚C, liquid Co and tungsten react to form an intermediate phase Co 7W6 which has a range of homogeneity between 70 and 76% W w18,19x. Moreover, numerous ternary species are formed in the system C᎐Co᎐W w20x. Besides this, the sample material is expected to be considerably changed in the ablation spot.
Calibration curves were linearised using internal standardization by tungsten. As an example, the calibration line ICo 228rI W 224 vs. %Co for mps 12 is presented in Fig. 4 Žcurve 3.. Regression equations using three and four parallel measurements Ž mps 12 and 16. for Co 228 nm, Co 230 nm, Co 236 nm and Co 238 nm analytical lines with W 224 nm as the internal reference line are presented in Table 5. For 50% of the regression lines the intercepts on the intensity axis are
Table 5 Statistical evaluation of calibration lines for Co in WC coatings a Co line Žnm.
r
⌬ cc Žwt.%.
100⌬ ccrcc Žrelative %.
b
sb
sb t0.05;
a
sa
ta
t0.05;
m=p
228 228 230 230 236 236 238 238
0.9976 0.9951 0.9978 0.9950 0.9966 0.9948 0.9962 0.9952
1.23 1.59 1.18 1.62 1.46 1.65 1.54 1.59
8.15 10.57 7.81 10.74 9.68 10.96 10.23 10.53
0.0174 0.0157 0.0164 0.0147 0.0582 0.0517 0.1004 0.0920
0.0004 0.0004 0.0004 0.0004 0.0015 0.0014 0.0028 0.0024
0.0009 0.0009 0.0008 0.0009 0.0034 0.0031 0.0062 0.0052
0.0009 0.0186 0.0066 0.0233 y0.0532 y0.0014 0.0125 0.0916
0.0062 0.0068 0.0056 0.0064 0.0246 0.0228 0.0449 0.0390
0.15 2.78 1.18 3.66 2.16 0.06 0.28 2.35
2.23 2.15 2.23 2.15 2.23 2.15 2.23 2.15
12 16 12 16 12 16 12 16
a Ablation for 10 min corresponds to 144 cycles, translation velocity was 1 mm sy1 . Range of percentages 8᎐22.9% Co, centroid of percentages c c s 15.08% Co, confidence limits about the centroid c1 c , c 2 c , confidence interval about the centroid ⌬ c c s c 2 c y c1 c , relative width of the confidence interval 100⌬ c crc c s 100Ž c2 c y c1 c .rc c , number of calibration samples m s 4, number of parallel ablations on each calibration sample ps 3, and ps 4; number of replicates considered for the calculation of the confidence interval about the centroid q s 3, coefficient of correlation r, value of Student distribution function t ␣ ; at the level of significance ␣ s 0.05 and degree of freedom s mpy 2, slope b, standard deviation of the slope sb , confidence limit about the slope sb t 0.05; , intercept on the intensity axis a, standard deviation of the intercept s a , calculated value of the Student test t a .
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in Fig. 5. Dashed hyperbolae Žband 1. correspond to the regression uncertainty, dotted hyperbolae Žband 2. are the limits of the dispersion band. Individual observations of signal ratios ICo 228r I W 224 corresponding to calibration samples are presented, too. The parameters of this linear regression are given in the second row of the Table 5. 3.9. Analysis of unknown sample Fig. 4. Calibration curves: 1, ICo 228rI W 224 vs. %Co at the fixed spot ablation, mps 24; 2, ICo 228 vs. %Co at the ablation during sample translation; 3, ICo 228rI W 224 vs. %Co at the ablation during sample translation, mps 12. Error bars represent "standard deviation.
not statistically significant. Differences between slopes obtained for mps 12 and mps 16 are not statistically significant during the period of 2 weeks. Finally, two confidence bands along the regression line were computed w21,22x. The confidence band of the true regression line was computed for 95% probability according to the equations c 2 u ,c1 u s c u " Ž t¨ ,0.05 sŽ y , x .rb . 1r Ž mp . q Ž c u y c c . 2rÝ Ž c i y c c . 2
1r2
Ž 3,4.
where c u is the percentage of unknown sample, c 2 u , c1 u are corresponding confidence limits, c c is the centroid of percentages and c i is the ith percentage with i s 1 to mp. This band is related to the uncertainty due to the use of the calibration graph. The second confidence band, called the dispersion band, was computed for individual observation of analytical signal corresponding to c u , according to equations
The WC᎐Co top coating sprayed on 20 Nir80 Al intermediate layer was analyzed both using the WDXrEDX method and the LA-ICP-AES at sample translation. Conditions for laser ablation analysis were the same as for calibration study. The electron microprobe technique yielded the value of Ž11.9" 0.6.% Co Žfour replicates. and the LA-ICP gave the result Ž12.5" 0.8.% Co based on three replicates. The ‘"’ values denotes the confidence interval limits.
4. Conclusions The results presented in this work suggest that laser ablation inductively coupled plasma atomic emission spectrometry ŽLA-ICP-AES. could be used for characterisation of tungsten carbide coatings containing cobalt. Homogeneity of plasma sprayed coatings was tested with the
c 2 u ,c1 u s c u " Ž t¨ ,0.05 sŽ y , x .rb . 1 q 1r Ž mp . q Ž c u y c c . 2rÝ Ž c i y c c . 2
1r2
Ž 5,6.
Both the confidence bands are drawn along the calibration line ICo 228rI W 224 vs. %Co for mps 12
Fig. 5. Calibration curve ICo 228rI W 224 vs. %Co obtained at sample translation, mps 12. Confidence bands: 1, regression band; 2, dispersion band.
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resulting relative standard deviation of the ratio CorW approximately 4%. The LA-ICP-AES signal ratio of CorW was found to be sensitive to variation of the gas mixture composition ŽH 2rAr. at sample spraying because the intensity ratio changes were statistically significant. Time correlation of signals of W and Co was proved to be useful for linearization of calibration curves of cobalt. Due to the relative width of the confidence interval about the centroid of calibration lines, which is approximately "5%, the determination of Co can be regarded rather semiquantitative. However, the X-ray-based techniques ŽXRF, EDXrWDX. yielded results with comparable precision. Improvements of precision could be expected for ablation of greater sample surface. This is the aim of our further studies.
Acknowledgements V.K. and V.O. gratefully acknowledge the Grant Agency of the Czech Republic for bestowing the financial support Žproject 203r97r0345. and thank the Ministry of Education and Youth of the Czech Republic for the grant to the project VS 97020. V.K. and V.O. thank J. Filipensky ´ and J. Ondracek ´ˇ from Plasmametal Co., Brno, Czech Republic and O. Ambroz ˇ from the Faculty of Mechanical Engineering of the Technical University Brno, Czech Republic, for the preparation of plasma-sprayed coatings. V.K. and V.O. thank P. Janosˇ and V. ˇ Sprta from the Research Institute of ´ ´ nad Labem, Czech ReInorganic Chemistry, Ustı public, for the XRF analyses of samples and P. Sulovsky ´ from the Department of Mineralogy, Petrology and Geochemistry of the Faculty of Science of the Masaryk University Brno, Czech Republic, for the EDXrWDX analyses of samples. The authors wish to thank Perkin-Elmer for the loan of the Optima 3000 DV ICP spectrometer.
References w1x M.M. Schwartz ŽEd.., Handbook of Structural Ceramics, Mc Graw-Hill, New York, 1992. w2x V. Kanicky, ´ I. Novotny, ´ J.L. Lacour, P. Mauchien, J.M. Mermet, J. Phys. 4 Ž1994. 710. w3x V. Kanicky, ´ J. Musil, J.M. Mermet, Appl. Spectrosc. 51 Ž1997. 1037. w4x V. Kanicky, ´ J. Musil, M. Benda, J.M. Mermet, Collect. Czech. Chem. Commun. 61 Ž1996. 1167. w5x V. Kanicky, ´ I. Novotny, ´ J. Musil, J.M. Mermet, Appl. Spectrosc. 51 Ž1997. 1041. w6x D. Potter, I. Abell, Anal. Sci. 7 Ž1991. 1239. w7x J.S. Becker, U. Breuer, J. Westheide, A.I. Saprykin, H. Holzbrecher, H. Nickel, H.J. Dietze, Fresenius’ J. Anal. Chem. 355 Ž1996. 626. w8x J. Musil, J. Therm. Spray Technol. 6 Ž1997. 387. w9x J. Musil, J. Fiala, Surface Coating Technol. 52 Ž1992. 211. w10x J. Musil, M. Alaya, R. Oberacker, J. Therm. Spray Technol. 6 Ž1997. 449. w11x K.A. Khor, P. Cheang, C.S. Yip, J. Therm. Spray Technol. 6 Ž1997. 109. w12x P. Filip, K. Mazanec, A.C. Kneissl, R. Melicharek, Z. ´ Metallkd. 88 Ž1997. 131. w13x Hardware Guide OPTIMA 3000, Ch. 2, Perkin Elmer Corp., Norwalk, CT, 1993, p. 5. w14x M. Hemmerlin, J.M. Mermet, M. Bertucci, P. Zydowicz, Spectrochim. Acta Part B 52 Ž1997. 421. w15x H. Remy, Lehrbuch der Analytischen Chemie, Band II, Akademische Verlagsgesellschaft Geest & Portig KG, Leipzig, 1961. w16x F.A. Cotton, G.W. Wilkinson, Advanced Inorganic Chemistry, John Wiley & Sons, New York, 1966. w17x G.V. Raynor, V.G. Rivlin, Phase Equilibria in Iron Ternary Alloys, The Institute of Metals, 1988. w18x J. Kubıcek, ´ˇ L. Ivan, Proceedings of the 1st Czech National Thermal Spray Conference, Brno, Czech Republic, 19᎐24 April 1994, p. 111. ˇ w19x M. Cepera, J. Zeman, J. Musil, P. Fiala, J. Filipensky, ´ Proceedings of the 1st Czech National Thermal Spray Conference, Brno, Czech Republic, 19᎐24 April 1994, p. 115. w20x G. Barbezat, E. Muller, B. Walser, Sulzer Tech. Rev. 4 ¨ Ž1988. 4. w21x J.M. Mermet, Spectrochim. Acta Part B 49 Ž1994. 1313. w22x D.L. Massart, B.G.M. Vandeginste, S.N. Deming, Y. Michotte, L. Kaufman, Chemometrics, a Textbook, Elsevier, Amsterdam, 1988.